Distraction osteogenesis (DO) is a means to elongate bone by placing tensile loads on a healing osteotomy. Although DO has been used on the mandible for only a decade, the technique is already supplanting more traditional orthognathic procedures, especially for children. Despite the fact that DO uses mechanics to accomplish its goals, the actual mechanical environment is unknown. Functions such as mastication must place intermittent compressive or shearing strains on the distraction site. These strains, as well as the tensile strains produced by the distraction appliance, are of critical biological significance for the osteogenic process, but have never been measured. Using a well-established model for mandibular DO, the pig, we propose (Aim 1) to clarify the mechanical environment of the distraction site at the time of osteotomy, during distraction, and during consolidation by making direct measurements of strain using strain gages, digital ultrasonics and differential variable reluctance transducers. These studies will investigate the influence of appliance size, stability, and placement as well as the action of specific muscles and occlusal loads. Further, we will address a major concern of treating growing children by mandibular DO, whether the procedure adversely affects growth at the mandibular condylar by overloading the TMJ. Thus, we propose (Aim 2) to ascertain the impact of DO on TMJ mechanics and growth by performing strain gage measurements on the condylar neck and relating the findings to growth, as measured by mineral apposition rate and periosteal deformation. The mechanical environment of DO, particularly the stability of the distraction site, may affect the course of healing and the final result by influencing which cells are recruited, the rapidity with which they proliferate, and their eventual fate. Therefore, we propose (Aim 3) to relate the mechanical history of DO to the repair process by assessing the contributions of the periosteum, muscle connective tissue, and vascular system to the regenerate. This will be accomplished by pulse labeling of replicating cells during distraction, combined with an investigation of protein expression at the time of sacrifice. The most important contributions of this proposal will be the characterization of strain at the DO site under natural conditions, and the application of that information to understanding of how DO works on a cellular level and how it affects mandibular growth.